====== What is corrosion? ======

Corrosion is usually defined as the deterioration of a metal or its properties caused by a reaction with its environment. Most metals occur naturally in the form of oxides and are usually chemically stable. When exposed to oxygen and other oxidizing agents, the refined metal will try to revert to its natural oxide state. In the case of iron, the oxides will be in the form of ferrous or ferric oxide, commonly known as rust. 

Metallic corrosion generally involves the loss of metal at a particular location on an exposed surface. Corrosion occurs in various forms ranging from a generalized attack over the entire surface to a severe concentrated attack. In most cases, it is impossible or economically impractical to completely arrest the corrosion process; however, it is usually possible to control the process to acceptable levels. 

The U.S. government funded a detailed study of the annual cost of corrosion in 1975. The total cost of metallic corrosion to the U.S. economy was estimated at 4% of the GNP ($70 billion dollars). Of that, approximately 30% was defined as avoidable. Adjustment based on today's current economy puts total current costs in the $300 billion range with over $100 billion of that avoidable. 

====== The process of Corrosion ======
Metallic corrosion is caused by the flow of direct current from one part of the metal surface to another. This flow of direct current causes the loss of metal at the point where current discharges into the environment (oxidation or anodic reaction). Protection occurs at the point where current returns to the metal surface (reduction or cathodic reaction. The rate of corrosion is proportional to the magnitude of the corrosion current. One ampere of direct current removes approximately twenty pounds of steel in one year. Where corrosion occurs and to what extent depends upon the environment to which the metal is exposed. 

Four conditions must be met for corrosion to occur. Elimination of any of the four conditions will halt the corrosion reaction.
  * **Anode** - the oxidation reaction occurs here. Current discharge into the environment and metal loss are associated with this reaction.
  * **Cathode** - the reduction reaction occurs here. Current acceptance and metal protection are associated with this reaction.
  * **Electrolyte** - the environment to which both the cathode and the anode are exposed. The electrolyte must have the capacity to conduct electrical current through the flow of ions.
  * **Metallic path** - the anode and the cathode must be connected via a metallic connection that conducts electrical current flow through the flow of electrons.


====== Causes of Corrosion ======

Corrosion is a natural process. The primary driving force of corrosion is based upon the transformation of iron from its natural state to steel. The refining of iron ore into steel requires the addition of energy. Steel is essentially an unstable state of iron and corrosion is the process of iron returning to its natural state. The energy used in the refining process is the driving force of corrosion.

Corrosion cells are established on underground pipelines for a variety of causes. A primary cause of corrosion is due to an effect known as galvanic corrosion. All metals have different natural electrical potentials. Where two metals with different potentials are connected to each other in a common environment, current will flow causing corrosion to occur. The coupling of steel to a different metal, such as copper, will cause a corrosion circuit to be established. Direct coupling of copper to steel will cause the steel to corrode much faster than normal. Another form of this is the coupling of rusty pipe to new, clean steel. The natural difference in potential causes the new steel to corrode more rapidly than the old steel. Other causes of pipeline corrosion cells include the effect of different soil types, oxygen availability, stray current interference and microbiological growth. 

Two other unique causes (and sometimes related) are stress and hydrogen.
  * Stress Corrosion occurs when even a very small pit forms in a metal under stress. The concentrated stress either deepens and extends the pit, or cracks the protective film which tends to form. Under continued exposure to the corrosive medium and stress, the crack extends by alternate corrosion and stress failure.
  * Hydrogen Embrittlement and hydrogen attack results when atomic hydrogen penetrates into the grain boundaries of steel producing microcracks, blistering and loss of ductility. The atomic hydrogen combines into molecules and results in blistering and laminations.

====== Forms of Corrosion ======

Corrosion exhibits itself in a number of ways. A brief description of some of these is provided below.
  * **General Corrosion** is the most common form of corrosion. It exhibits itself in an overall attack of the metal surface with no apparent concentrations. An example is the effect of atmospheric corrosion.
  * **Pitting Corrosion** results in a localized, concentrated attack and has the appearance of holes or craters.
  * **Crevice Corrosion** occurs in shielded areas where stagnant corrosive electrolyte accumulates. This type of corrosion occurs under bolt heads, gasket surfaces, and overlapping metal connections.
  * **Erosion-Corrosion** is a combination of electrochemical and mechanical damage that occurs in environments of high fluid velocities or mechanical movement between two metals.
  * **Selective Leaching** results in one constituent of an alloy being selectively removed, leaving a porous replica of the original alloy. An example is the dezincification of brass or bronze and the graphitization of cast iron where iron is removed selectively, leaving a replica composed of carbon or graphite.

====== How to control corrosion ======

The five general methods used in the control of corrosion are coating, cathodic protection, material selection, environmental modification, and design practices. Control of underground corrosion is primarily achieved by two methods: coating and cathodic protection. An effective external coating can provide corrosion protection to over 99% of the exposed pipe surface. The protective coating is usually applied to the pipe or tank before burial. The coating serves to electrically insulate the metal from the soil. If the metal could be completely isolated, then the establishment of corrosion cells would be prevented and no corrosion current would flow. However, no coating can be considered a perfect coating. Damage to the coating as a result of handling, transportation, installation, thermal stresses, and soil stresses will eventually create defects or "holidays" that expose the underlying steel to the environment.

Cathodic protection is an electrochemical technique for preventing corrosion of a metal exposed to an electrolyte. The process involves application of DC electrical current to the metal surface from an external source. By forcing the metal surface to accept current from the environment, the underground metal becomes a cathode and protection occurs. The external source can use outside AC power through a rectifier and groundbed or by attaching sacrificial metals such as magnesium or aluminum to the structure to be protected. It is used extensively in preventing corrosion to underground and submerged steel structures; such as pipelines, production well casings, and tanks.

Effective application of cathodic protection can provide complete protection to any exposed areas for the life of the structure. The combination of an external coating and cathodic protection provides the most economical and effective choice for protection of underground and submerged pipelines. For bare or ineffectively coated existing pipeline systems, cathodic protection often becomes the only practical alternative for corrosion protection.

Cathodic protection is a mandated requirement of federal and state regulations governing underground transmission pipelines, gas distribution systems, and underground fuel tanks. These requirements include installation, monitoring, and maintenance of cathodic protection systems. 

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